Most existing mobile communication systems incorporate a cellular architecture in which geographical areas are divided into roughly equal sized cells. Each cell contains a base station for interfacing mobile units in the cell to a mobile switching center (MSC) and a backbone wired network. The MSC connects the base stations together for communication therebetween. A base station is equipped with radio transceivers that enable rf communications over available frequency channels with mobile units in the cell.
Techniques for determining the location of mobile stations exist in the prior art. One known method involves using a global positioning system (GPS). To determine the location of a mobile station, GPS requires that the mobile station be in the field of view of a satellite. Mobile stations travel through tunnels and into buildings and many other areas where satellite signals cannot penetrate. Thus, when a mobile station is in one of these areas, GPS cannot determine the location of the mobile station. Furthermore, mobile stations need to communicate GPS location information to the network requiring use of rf channels.
Another method directly measures radio signals traveling between a set of base stations and a mobile station and then back to the base stations. Time or phase measurements obtained by such a trilateration method are used to determine the length or direction of the radio path. This requires expensive systems at the base station and precise synchronization across base stations to obtain reliable location data.
Recently, a highly accurate method for estimating the position and velocity of mobile stations has been developed which is described in the article entitled "Estimating Position and Velocity of Mobiles in Cellular Radio Networks" by Hellebrandt et al. in IEEE Transactions on Vehicular Technology, Vol. VT-26, No. 1, pp. 7-11, February 1997. According to this technique, the actual downlink strength of six adjacent base stations is measured by mobile stations at predefined locations. Profiles are developed for the predefined locations and stored in a database. Then, the average signal strength of the six base stations is measured by the mobile station at its current location. The best squares are estimated and compared with the profiles stored in the database. The point in the cell providing the minimum least-square error and satisfying the transformed signal data is considered as the location of the mobile station. Here again, the location information which is known to the mobile station needs to be conveyed to the network via wireless channels.